Many printing apparatuses are controlled by a pulse signal or firing signal which causes a printing substance, such as ink for example, to be applied to a print medium such as paper. For instance, an ink jet printing apparatus may include a printhead having printing elements that are controlled by a signal. In particular, the printhead can comprise an ink reservoir and an ink ejection chip with nozzles and corresponding printing elements or ink ejection actuators, such as heaters. In such printing devices, signals are supplied that cause the heaters to heat the ink held in a chamber at the nozzles which in turn causes the ink to be ejected from the nozzles onto the print medium at selected ink dot locations within an image area. A carrier moves the printhead relative to the medium, while the ink dots are jetted onto selected pixel locations.
Users of printing apparatuses continue to demand higher quality images and text which requires higher resolution, or, in other words, that more dots be printed per unit area. Users also continue to demand higher print speeds, such that pages can be printed faster. One way to achieve higher resolution and higher speeds is to include smaller components, such as smaller ink actuators and nozzles which create the dots, and to operate these components at faster speeds. However, as ink actuators become smaller, they require less energy in order to nucleate the ink and cause it to be ejected onto the print media. Therefore, these components are more sensitive to energy, and if excess energy begins to build up in the system, the components may cause ejection of ink at undesired times. Accordingly, excess energy needs to be controlled to permit correct printing, particularly for high speed and high resolution printing where actuators are more sensitive.
Excess energy can build up over time due to various factors. For instance, excess energy may build up due to the transitions of current flow between on and off states under control of the signals that control the actuators. In particular, in a thermal ink jet printing apparatus, the actuators comprise heating elements that can be controlled by a firing pulse that allows current to be turned on to create the heating effect (and cause the ejection of the printing substance) and then turned off to stop the heating effect (and stop any additional ejection). The dissipating current as the heater is turned off can cause build up of energy, energy that is not needed but is a result of the transitioning process. Therefore, turning off the current flow to the heater in a rapid manner is desired. On the other hand, a heating element cannot be turned off too fast because rapid changes in that current and/or in the firing pulse that causes that current can cause excess voltages to appear in the system, due to inductances of the circuitry and components. These excess voltages or back EMF can cause damage to circuit components and to heaters if they exceed a certain level.
Therefore, it is desired to precisely control the speed the speed of these transitioning signals such that they are 1) not too fast so as to cause back EMF damage, 2) fast enough so that high resolution and speed can be achieved, and 3) not too slow such that excess energy builds up and causes untimely firing of the heater, decreased component life, and other problems.
The temperature of a printing apparatus can vary widely during operation. The firing of heaters or other actuators can cause build up of heat which can affect the performance of the circuit components. This can cause variances in the amount of control over the speed at which signals are turned on and off. As mentioned above, controlling these transition speeds at precise levels is important for proper operation and to prevent damage. Accordingly, it is also desired to provide methods and systems that accurately control the timing of the printer signals across a broad range of operating temperatures. It is further desired to control such signals utilizing circuit components which are not difficult to implement.